So-called “impregnated” drag bits have been used conventionally for drilling rock formations that are hard, abrasive, or both. More particularly, conventional earth boring drag bits with diamond-impregnated cutting structures, commonly termed “segments,” or, alternatively, discrete diamond-impregnated cutting structures have been employed to bore through very hard and abrasive formations, such as basalt, dolomite, and hard sandstone. These conventional impregnated drag bits typically employ a cutting face comprising a diamond impregnated material, which refers to an abrasive particle or material, such as natural or synthetic diamond grit, uniformly dispersed within a matrix of surrounding material. As a conventional impregnated drag bit drills, the matrix wears to expose the abrasive particles, the abrasive particles also wear, and worn abrasive particles may be lost and new abrasive particles, which were previously surrounded by matrix material, may be exposed.
In fact, many conventional diamond impregnated segments may be designed to release, or “shed,” such diamonds or grit in a controlled manner during use of the drag bit. As a layer of diamonds or grit is shed from the face, underlying diamonds are exposed as abrasive cuttings and the diamonds that have been shed from the drag bit wear away the exposed continuous phase of the segment in which the interior diamonds are substantially uniformly dispersed until the entire diamond-impregnated portion of the bit has been consumed. Thus, drag bits with diamond-impregnated segments may maintain a substantially constant boring rate or rate of penetration, assuming a homogeneous formation, as long as diamonds remain exposed on such segments.
Regarding conventional abrasive-impregnated cutting structures, the abrasive material with which the continuous matrix material is impregnated preferably comprises a hard, abrasive and abrasion-resistant particulate material, and most preferably a super-abrasive material, such as natural diamond, synthetic diamond, or cubic boron nitride.
The impregnated segment may include more than one type of abrasive material, as well as one or more sizes or quality grades of abrasive material particles. In conventional abrasive-impregnated cutting structures, the abrasive is substantially homogeneously distributed (i.e., not segregated) within the continuous matrix material. The continuous matrix material may be chosen for wettability to the abrasive particles, mechanical properties, such as abrasion resistance, or both, and may comprise one or more of copper, a copper-based alloy, nickel, a nickel-based alloy, cobalt, a cobalt-based alloy, iron, an iron-based alloy, silver, or a silver-based alloy.
Two general approaches are conventionally employed to fabricate drag bits having abrasive-impregnated cutting structures.
In a first approach, an abrasive-impregnated cutting structure may be cast integrally with the body of a drag bit, as by low-pressure infiltration. For instance, one conventional abrasive-impregnated cutting structure configuration includes placing abrasive material into a mold (usually mixed with a molten wax) as by hand-packing, as known in the art. Subsequently, the mold may be filled with other powders and a steel core and the entire assembly heated sufficiently to allow the infiltrant, such as a molten alloy of copper or tin to infiltrate the powders and abrasive material. The result, upon the infiltrant cooling and solidifying, is a bit body, which has abrasive-impregnated cutting structures bonded thereto by the continuous matrix of the infiltrant.
In a second approach, the abrasive-impregnated cutting structures may be preformed or fabricated separately, as in hot isostatic pressure infiltration, and then brazed or welded to the body of a drag bit. Thus, conventional abrasive-impregnated cutting structures may be formed as so-called “segments” by hot-pressing, infiltration, or the like, which may be brazed or otherwise held into a bit body after the bit body is fabricated. Such a configuration allows for the bit body to include infiltrants with higher melting temperatures and to avoid damage to the abrasive material within the abrasive-impregnated cutting structures that would occur if subjected to the higher temperatures.
In a third process preformed segments are placed in the mold and then matrix added and infiltrated as in example one above.
In a fourth process encapsulated grit is dispersed within the matrix, etc. and then cast as example one mentioned above.
As known in the art, diamond impregnated segments of drag bits may be typically secured to the boring end, which is typically termed the “face,” of the bit body of the drag bit, oriented in a generally radial fashion. Impregnated segments may also be disposed concentrically or spirally over the face of the drag bit. As the drag bit gradually grinds through a very hard and abrasive formation, the outermost layer of the impregnated segments containing abrasive particles wear and may fracture, as described above. For instance, U.S. Pat. No. 4,234,048 (the “'048 patent”), which issued to David S. Rowley on Nov. 18, 1980, discloses an exemplary drag bit that bears diamond-impregnated segments on the crown thereof. Typically, the impregnated segments of such drag bits are C-shaped or hemispherically shaped, somewhat flat, and arranged somewhat radially around the crown of the drag bit. Each impregnated segment typically extends from the inner cone of the drag bit, radially outwardly therefrom and up the bit face to the gage. The impregnated segments may be attached directly to the drag bit during infiltration or partially disposed within a slot or channel formed into the crown and secured to the drag bit by brazing.
Alternatively, conventional discrete, post-like cutting structures are disclosed in U.S. Pat. Nos. 6,458,471 and 6,510,906, both of which are assigned to the assignee of the present invention and each of the disclosures of which are incorporated, in their entirety, by reference herein.
U.S. Pat. No. 3,106,973 issued to Christensen on Oct. 15, 1963, discloses a drag bit provided with circumferentially and radially grooves having cutter blades secured therein. The cutter blades have diamond impregnated sections formed of a matrix of preselected materials.
U.S. Pat. No. 4,128,136 issued to Generoux on Dec. 5, 1978, discloses a diamond coring bit having an annular crown and inner and outer concentric side surfaces. The inner concentric side surface of the crown defines a hollow core in the annular crown of the bit for accommodating a core sample of a subterranean formation. The annular crown is formed from a plurality of radially oriented composite segments impregnated with diamonds radially and circumferentially spaced apart from each other by less abrasive spacer materials.
U.S. Pat. No. 6,095,265 to Alsup discloses an adaptive matrix including two or more different abrasive compositions in alternating ribs or in staggered alternating zones of each rib to establish different diamond exposure in specified areas of the bit face. Alsup further discloses that the abrasive compositions for adaptive matrix bits contain diamond and/or other super-hard materials within a supporting material. The supporting material may include a particulate phase of tungsten carbide and/or other hard compounds, and a metallic binder phase of copper or other primarily non-ferrous alloys. Alsup discloses that the properties of the resulting metal-matrix composite material depend on both the percentage of each component and the processing that combines the components. Further, Alsup discloses that the size and type of the diamonds, carbide particles, binder alloy or other components can also be used to effect changes in the overall abrasive or erosive wear properties of the abrasive composition. Additionally, such adjacent “hard” and “soft” ribs may purportedly facilitate fluid cleaning in and around the ribs.
U.S. Pat. No. 6,458,471 to Lovato et al., assigned to the assignee of the present invention and the disclosure of which is incorporated herein its entirety by reference thereto, discloses cutting elements including an abrasive-impregnated cutting structure having an associated support member, wherein the support member is securable to an earth boring rotary-type drag bit body and provides mechanical support to the cutting structure.
U.S. Pat. No. 6,742,611 to Illerhaus et al., assigned to the assignee of the present invention and the disclosure of which is incorporated herein its entirety by reference thereto, discloses a first cutting element segment formed of a continuous-phase solid matrix material impregnated with at least one particulate super abrasive material, the first cutting element segment juxtapositioned with at least one second cutting element segment formed of a continuous-phase solid matrix material to form a laminated cutting element. Preferably, the at least one second cutting element segment is essentially devoid of impregnated super abrasive or abrasive particles. Alternatively, the at least one second cutting element segment can be impregnated with a preselected, secondary, particulate super abrasive material that results in the at least one second segment being less abrasive and less wear resistant than the at least one first abrasive segment.
While the above-discussed conventional abrasive-impregnated cutting structures and drag bits may perform as intended, it may be appreciated that improved abrasive-impregnated cutting structures and drag bits would be desirable. Further, it would be desirable to improve abrasive-impregnated cutting structures that exhibit selectable wear characteristics.
FIG. 1 is a section view through a diamond impregnated bit that illustrates a new condition of diamonds 10 encapsulated by a coating 12 and disposed in a matrix that includes a binder 14 that further includes hard particles such as carbide 16. In operation the matrix 14 wears away as does the encapsulating material 12. As shown in FIG. 2 at 18 the encapsulating material 12 wears faster than the matrix 14 ahead of the diamond 10 that now has one or more facets 20 on the leading face in the cutting direction exposed. FIG. 3 is a close up of the view in FIG. 2 and graphically illustrates the problem. The encapsulating material 12 is generally less wear resistant than the matrix 14 with the distributed carbide material 16 and as a result begins to form a deepening valley 22 caused by abrasive wear with the formation being drilled. The effect on the trailing portion of the diamond 10 such as near surface 24 is less pronounced but still some of the encapsulating material 12 has also worn off as surface 24 becomes exposed to the formation. On the trailing side of diamond 10 in the direction of rotation a wear pattern known as “comet tails” 26 because of its appearance is evident and better seen from a bottom view of a bit face shown in FIG. 4. As shown in FIG. 3 because of the close diamond 10 spacing, the comet tail from one diamond 10 can contribute to the removal of the encapsulating material 28 from adjacent diamond 10′ that is behind it measured in the direction of bit rotation indicated by arrow 30.
What is illustrated in the above FIGS. is that the encapsulation 12 for the diamonds 10 which is generally less abrasion and impact resistant than the matrix 14 that surrounds it, progressively undermines the structural support for the diamond 10 in the matrix 14. While the diamonds 10 are far more wear resistant and durable than the matrix 14 their ability to drill hard formations is limited to the physical support the diamonds 10 have in the matrix 14. If such structural support for the diamonds 10 is undermined by removal of the surrounding encapsulating material 12 then the impact with the formation simply breaks the diamond particle 10 with its remaining coating 12 right out of the matrix 14 prematurely. It should be noted that the desired wear process is for enough matrix 14 to wear so that at some point before the diamond 10 is fully consumed it is dislodged to allow other diamonds 10 deeper in the matrix 14 to come to the face and take over cutting through the hard formation. The problem is in optimizing the duration of time that the diamond 10 is retained in the matrix 14 so that it can support an optimum rate of penetration. Another issue is optimizing the diamond protrusion for increased effective depth of cut (DOC) and increased rate of penetration (ROP).
In the past diamond grit has been encapsulated in a single layer of tungsten carbide and binders. The carbide layer was an agglomerate of small WC particles which are agglomerated to the diamond grit in a rolling or tumbling process. The added organic binder is burned out during this process. The particle size of the agglomerate could be uniform or varied but it forms a single layer of a desired thickness. The shortcomings of such a single layer encapsulation have been discussed above and are the focus of the issue addressed by the present invention.
The present invention addresses this issue with a preferred design that features application of multiple layers that have either discrete or blended boundaries on the diamond that get progressively softer and less resistant to abrasion as they are applied to the diamond. Optionally, the matrix can be made somewhat softer by a reduction if not total elimination of added carbide particles in the matrix. It has been observed that bits are frequently pulled for replacement before the diamonds have been fully consumed. What can happen is that the rate of penetration falls off when enough encapsulated diamonds come off leaving only the matrix with carbide particles to make further progress into the formation while there are still other diamonds that have been exposed for further progress in drilling. The thinking is to make the matrix somewhat softer while using the layered encapsulation to get more run time with the encapsulated diamonds before they are knocked off the bit will improve the total footage and rate to the point of bit replacement. Those skilled in the art will appreciate that the description of the preferred embodiment with the associated figures will further point out other aspects of the invention while recognizing that the full scope of the invention is to be found in the appended claims.